A gas turbine engine is one example of a rotary machine having an axially extending compression section which is disposed about an axis R of the engine. The gas turbine engine has a combustion section and a turbine section downstream of the compression section. These sections are disposed about the axis R. An annular flowpath for working medium gases extends axially through the sections of the engine.
The working medium gases are compressed and diffused in the compression section. Fuel is mixed with the working medium gases in the combustion section and burned to add energy to the gases. The hot, pressurized gases are expanded through the turbine section to develop propulsive thrust and, through one or more turbines to extract energy from the gases by driving the turbines about the axis of the engine.
A rotor shaft extends axially in the engine to rotatably attach components of the compression section to the turbines. As each turbine is driven about the axis R by the expanding working medium gases, the turbine drives rotating components in the compression section about the axis. These rotating components in the compression section do work on the incoming gases to pressurize the gases.
In a turbofan gas turbine engine, the compression section may have three compressors in axial alignment for increasing the pressure of the incoming gases. The compressors are commonly referred to as the fan compressor, the low pressure compressor, and the high pressure compressor.
Each compressor has an outer wall and an inner wall which bound the working medium flowpath. The rotating components include arrays of rotor blades which extend outwardly across the working medium flowpath into proximity with the outer wall. Alternating with arrays of rotor blades are arrays of compressor vanes. Each compressor vane has an airfoil which extends radially inwardly between the outer wall and the inner wall, across the flowpath for working medium gases. Each airfoil of the vane adjusts the angular velocity component of the working medium gases as the gases exit the rotor stages and before the gases enter the adjacent rotor stage or a diffuser region of the compressor.
Such constructions are very different from turboprop constructions of the type shown in U.S. Pat. No. 2,934,150, issued to Fink entitled "Pressure-Contoured Spinner". Turboprop constructions do not have an outer wall which extends circumferentially about the propeller. And, the aerodynamic design of a propeller is very different from the aerodynamic design of an airfoil for a compressor which is surrounded by an outer wall.
Fink shows a contoured inner wall having an indentation to reduce drag at the juncture of the propeller with the spinner. Fink states this concept is equally adaptable to turbomachinery blading in order to alleviate local flow separation. However, there are significant aerodynamic performance differences between a turboprop with its array of airfoils only bounded by an inner wall (an unbounded cascade) and those which are bounded by an inner wall and an outer wall (bounded cascade).
With regard to bounded cascades, there are numerous examples of prior art in which the inner wall or the outer wall is contoured for aerodynamic considerations. These aerodynamic considerations include, for example, the aerodynamic efficiency of the airfoils, the flow losses experienced by the gases as the gases pass through the array of airfoils, and the choke flow characteristic of the array. The choke-flow characteristic is the level of pressure ratio across an array of airfoils above which an increase in pressure ratio does not increase flow through the array.
Examples of bounded cascade constructions are shown in U.S. Pat. No. 2,735,612 issued to Hausammann entitled "Blade Passage Construction for Compressors and Diffusers" which has projections into the flowpath for working medium gases resulting in a concave-convex wall path adjacent to the airfoil. U.S. Pat. No. 2,846,137 issued to Smith entitled "Construction for Axial-Flow Turbo Machinery" has a convex-concave shape or a concave-convex shape with respect to the flowpath at the end walls of the airfoils. U.S. Pat. No. 2,918,254 issued to Hausammann discloses a projection extending on the end wall from the pressure side to the suction side of the airfoil, the projection extending in the rearward direction. U.S. Pat. No. 2,955,747 issued to Schwaar entitled "Supersonic Axial Compressors" discloses adjacent rotor stages having the end walls of the adjacent stages angled with respect to each other. U.S. Pat. No. 4,371,311 issued to Walsh entitled "Compression Section for an Axial Flow Rotary Machine" has end walls that are curved with respect to the rotor and stator stages to form concave-convex regions at the end wall with respect to the flowpath upstream and downstream of the airfoil stage. German Patent Number 579989 "Blading of Axially Loaded Steam or Gas Turbines Without Head Rings" discloses end walls having angled flowpaths with either a concave or convex region at the leading edge of the airfoil. United Kingdom Patent 596,784 entitled "Improvements in and Relating to Elastic Fluid Turbines" discloses airfoils having a curved end wall.
These constructions for bounded cascades illustrate the many uses of curved surfaces extending in the axial direction along the end wall to influence the flow characteristics of an array of airfoils. The above art notwithstanding, scientists and engineers working under the direction of Applicants assignee have sought to develop airfoils which have increased efficiency and reduced aerodynamic losses in airfoil regions adjacent the walls which bound the flowpath for working medium gases.